Asphalt paving material is generally comprised of a mixture of asphalt cement and various size fractions of aggregate materials. Generally such mixture is produced in an asphalt production plant at a temperature within the range of 250-350° F. Paving of a roadway with this hot asphalt paving material is generally carried out by an asphalt paving machine that is supplied with asphalt paving material by a number of supply trucks and/or a material transfer vehicle. A typical asphalt paving machine is self-propelled and driven by a wheeled or tracked drive system. In most such machines, a hopper is located at the front end of the machine to receive asphalt paving material from a truck or material transfer vehicle, and a conveyor system located below the hopper transfers the asphalt paving material from the hopper to a transverse distributing auger assembly that is mounted near the rear of the machine. The transverse distributing auger assembly deposits the asphalt paving material onto and across the roadway or other surface to be paved. A floating screed located at the rear end of the machine behind the distributing auger assembly includes one or more screed plates that directly contact the asphalt paving material adjacent to the distributing auger assembly. The screed assembly imparts a compacting force to the asphalt paving material on the surface to form an asphalt mat.
It is important that the temperature of the asphalt paving material be maintained above a known level (depending on the mix design, ambient temperature and other factors) during the transportation and paving process in order for the asphalt paving material mixture to be properly compacted on the roadway or other surface being paved. Consequently, the screed plates of the floating screed are typically heated to a temperature within the range of 300-400° F. to prevent the asphalt cement portion of the asphalt paving material from congealing or otherwise adhering to the lower surfaces of the screed assembly. Some screed plates are heated by oil or gas burners mounted adjacent to the top side of the screed plates so that the flames from the burners impinge upon the top surfaces of the screed plates. These systems sometimes apply intense heat to localized portions of the screed plates, resulting in uneven heating of the screed plates that allows the asphalt cement to stick to the bottom of the insufficiently heated portions of the screed. In addition, if such a heating system is not carefully controlled, the portions of the screed plate that are overheated may become warped. Furthermore, direct flame heating systems emit smoke and fumes that create unpleasant and even unsafe working conditions for the operator of the screed assembly. Other screed plate heating systems employ a heater which is adapted to heat thermal fluids contained within a reservoir that is mounted directly on top of the screed plates. These systems may provide uniform heating of the screed plates, but they require that the thermal fluid circuit be pressurized to a relatively high pressure, which increases the cost and complexity of the heating system and may result in leaks. Still other screed plate heating systems employ electrical heating elements or coils that directly or indirectly apply heat to the screed plates. Such electrical systems may produce uneven heating similar to that obtained by direct flame heating systems because of the difficulty in maintaining uniform contact between the heating elements and the upper surfaces of the screed plates, especially as the screed twists and flexes to maintain grade across the width of the surface being paved.
In addition to the disadvantages described herein, many of the conventional heating assemblies for heating screed plates are relatively complex and require significant labor and time to make adjustments or to effect repairs. It would be desirable therefore, if a relatively simple heating system could be provided that would provide consistent uniform heat to all parts of the screed plates and would be easy to operate and maintain.